In general, the present invention relates to the packaging of food products, particularly foods such as, for example, meat or fish, which may release blood or other liquids.
In particular, the invention relates to a tray comprising a sheet of substantially open-cell, low-density polystyrene having the capacity to absorb liquids, particularly aqueous liquids, and to a method of producing it.
In the packaging of foods which may release blood, such as meat or fish, there is a problem in providing packages which can isolate and hide the blood released by the food, since it has been found that is presence and visibility inside the package renders the latter unattractive to the purchaser.
Amongst the solutions proposed for this problem in the prior art, the simplest consists of the provision of a tray provided with a pad or layer of absorbent material, generally paper. Solutions of this type are described, for example, in patent applications EP-A-O 182 139, GB-A-1 168 925, EP-A-0 544 562, in patent FR 2 688 474, and in utility model DE 9013898.8.
The absorbent layer may simply be fixed to the internal surface of a tray of plastics material or interposed between two sheets of plastics material, of which that which is in contact with the food has holes through which the liquid released by the food is conveyed towards the interposed absorbent layer.
The trays mentioned above have the disadvantage of being made of materials of different kinds, that is, expanded plastics, generally polystyrene, and paper or similar hydrophilic materials, which are difficult to separate from one another so that there is little or no chance of their being recycled. Moreover, owing to the presence of the paper, the production cost of these trays is considerably greater than that of conventional non-absorbent trays.
Trays made of a single material, generally expanded polystyrene, which can isolate the liquid exuded from the food, causing it to descend by gravity into a space formed between two sheets of the aforesaid plastics material by passing through holes formed in the sheet on which the food is placed are also known. Examples of these trays are given in patent applications EP-A-0 574 819 and WO 94/00366.
Trays of the type described above have the disadvantage of necessarily having quite large holes in order to allow the blood to descend by the gravity and, for this reason, leave the blood which has collected in the space easily visible. Moreover, the blood can easily return the sheet which is in contact with the food as a result of the inversion or simply the inclination of the tray.
The technical problem upon which the present invention is based is that of providing a tray which is made solely of plastics material and which itself has the capacity to absorb liquids, particularly aqueous liquids, released by foods, thus preventing the problems set out above with reference to the trays of the prior art.
A partial solution to the aforementioned technical problem was offered in patent application EP 0 090 507, which describe a xe2x80x9cfast foodxe2x80x9d container produced from a composite structure constituted by two superimposed sheets of which one is a normal sheet of closed-cell expanded plastics material and the other is a sheet of substantially open-cell plastics material which has the ability to absorb condensed steam coming from the foods present in the container.
The sheet of substantially open-cell plastics material constitutes the inner layer of the container which is in direct contact with the food.
The aforesaid open-cell sheet has a high density (150-450 g/l), however, and its ability to absorb water is not adequate for the purposes of the present invention.
The open-cell sheet produced according to the method of the aforementioned patent application also has an irregular surface which impairs the appearance of the tray to a certain extent.
According to application EP 0 090 507, the open-cell sheet is produced, by the mixing of polystyrene with an excess of a chemical nucleating and expanding agent such as sodium bicarbonate/citric acid, in an extruder having a single mixing chamber and a single screw, the molten mixture being brought to a temperature of 196-204xc2x0 C. immediately before extrusion.
The technical problem set out above is solved, according to the present invention, by a method of producing substantially open-cell polystyrene sheet, comprising the steps of:
forming a mixture comprising from 30 to 95% of polystyrene incorporating an aliphatic hydrocarbon with 4-6 carbon atoms, from 0 from 65% of polystyrene, and from 0.2 to 10% of a nucelating agent,
mixing the mixture obtained and melting it by heating inside an extruder,
bringing the mixture to a temperature of 130-150xc2x0 C. in the final portion of the extruder,
extruding the mixture in a lower-pressure atmosphere to produce an expanded sheet.
Simply xe2x80x9cpolystyrenexe2x80x9d means polystyrene as such, that is, not incorporating hydrocarbons.
The percentage is given herein should be understood as the weight relative to the total weight of the mixture if not specified otherwise.
In order to increase the absorption capacity of the sheet, a quantity of from 5 to 10% by weight of the aforesaid nucleating agent, relative to the total weight of the mixture, is preferably used.
The aforesaid mixture preferably has a content of from 1 to 7% by weight, relative to the weight of the mixture, of aliphatic hydrocarbon with 4-6 carbon atoms and the hydrocarbon is preferably pentane.
The density of the final expanded sheet can be modified by variation of the percentage of polystyrene and, in particular, increases with increases in the aforesaid percentage.
Moreover, there are various types of polystyrene incorporating aliphatic hydrocarbons with 4-6 carbon atoms on the market, which incorporate different quantities of hydrocarbon; the density of the sheets produced from these products decreases as the hydrocarbon content increases.
The aforesaid nucleating agent is preferably selected from the group comprising sodium bicarbonate, citric acid, talc, sodium carbonate, Hydrocerol(copyright) and gypsum. This nucleating agent need not necessarily have chemical expanding properties, whereas this constituted an essential condition in the method according to EP-A-0 090 507.
The aliphatic hydrocarbon with 4-6 carbon atoms is preferably constituted by pentane.
In order to give the sheet according to the invention a better capacity to absorb aqueous liquids, a quantity of at least one surfactant variable from 0.2 to 10%, and preferably from 1 to 4% of the total weight of the mixture is added to the starting mixture.
The at least one surfactant may be selected from the anionic, cationic and non-ionic surfactants in common use, and is preferably a salt of a sulphonic acid of formula Rxe2x80x94SO3H or of a sulphuric ester of formula Rxe2x80x94OSO3H, in which R is selected from the group comprising alkyl and alkylaryl, with an alkali metal or an alkaline-earth metal.
A surfactant which has been found particularly useful for the purposes of the present invention is the aliphatic sulphonate marketed by the firm NOVACROME with the name HOSTASTAT SYSTEM E 3904(copyright).
It is advantageous, in order further to increase the capacity of the sheet to absorb aqueous liquids, to add a quantity of up to 30%, preferably from 10% to 20%, of the mixture, of a shock-resistant polystyrene, preferably a dispersion of polybutadiene in polystyrene such as, for example, ENICHEM""s product EDISTIR SR 550(copyright).
In comparison with the closest prior art (EP-A-0 090 507), the present method has the advantage of providing a substantially open-cell sheet with a considerably lower density, for a given percentage of open cells, which can therefore absorb a larger volume of liquid for a given weight.
Moreover, the method according to the present invention is carried out at a lower temperature than the aforementioned method of the prior art (130-150xc2x0 C. instead of 196-204xc2x0 C.): this has the favourable consequence of a considerable energy saving, an improvement in the aesthetic characteristics of the skin, and better workability of the sheet.
The density of the expanded sheet can easily be adjusted upon the basis of the quantity of polystyrene resin incorporating pentane included in the starting mixture and can be reduced to values of 50 g/l when the polystyrene resin used in the starting mixture is constituted exclusively by polystyrene resin incorporating aliphatic hydrocarbon with 4-6 carbon atoms.
The present invention also relates to a polystyrene tray comprising a base and side walls, made from a sheet of substantially open-cell expanded polystyrene covered integrally on both of its surfaces by a film made of plastics material which defines the inner surface and the outer surface of the tray, respectively, holes with diameters of from 0.1-1.5 mm extending through the inner surface into the thickness of the sheet, wherein the expanded polystyrene sheet has a density less than or equal to 140 g/l and a capacity to absorb aqueous liquids greater than or equal to 4 g/dm2 per mm of thickness and is obtainable by the method described above.
The aforementioned plastics material of which the film is made is preferably selected from the group comprising polystyrene, polyethylene, polypropylene and copolymers thereof.
The film is advantageously opacified to prevent the liquid held in the underlying absorbent sheet from being visible. Opacification can be achieved by the incorporation, in the plastics material, of titanium dioxide or coloured lacquers constituted, for example, by dyes adsorbed on aluminum hydroxide, which generally incorporates small percentages of basic aluminium sulphate.
The high absorption capacity of the expanded sheet according to the present invention is due to its very high percentage of open cells and to the creation of countless receptacles as a result of the rupture of a certain percentage of cells, and also of the so-called xe2x80x9cribsxe2x80x9d, as can be seen from FIG. 1 which is an electron-microscope photograph of a cross-section of the sheet, enlarged 100 times, showing countless frayed and torn portions both of the cell membranes and of the xe2x80x9cribsxe2x80x9d.
The high absorbtion capacity of the sheet is inevitably accompanied by inadequate mechanical strength which causes problems with its use as the sole starting material for producing packaging trays.
In order to produce trays having adequate mechanical strength, it is therefore necessary to couple the sheet, on both of its surface, with a film made of plastics material, for example, polystyrene, before the thermoforming of the tray is carried out.
This coupling can be achieved by hot lamination or by means of adhesives, or by any other method conventionally used for this purpose.
The coupling of the sheet of substantially open-cell, expanded polystyrene with the film made of plastics material produces a sheet with a sandwich structure. Holes with diameters of 0.1-1.5 mm are formed in one of the surfaces of the sheet, extending into the intermediate layer constituted by the expanded sheet and, finally, trays are formed therefrom by thermoforming.
Alternatively, the holes may be formed in the base of the tray produced by the thermoforming of the sheet having a sandwich structure.
The trays thus obtained have an optimal appearance since the surface film of plastics material has a smooth surface without streaks or corrugations.
Moreover, the water-repellency of the sheet prevents the blood exuded from the food from wetting the inner surface of the tray creating unpleasant marks, and facilitates its penetration into the underlying absorbent layer through the holes.
By virtue of its low density and hence low weight, the open-cell expanded polystyrene sheet produced by the method described above can also be used as an absorbent layer in place of the paper layers or pads used in trays of the prior art.
In this case, in fact, the absorbent layer is required solely to have a good capacity to absorb aqueous liquids since the mechanical strength of the tray depends solely upon the mechanical properties of the plastics material of which the tray supporting the absorbent layer is made.
The use of the substantially open-cell expanded sheet produced by the method described above as an absorbent pad for conventional polystyrene trays also solves the problem connected with the difficulty in recycling the materials and a considerable reduction in costs is achieved.
To facilitate the absorption of liquids, the absorbent pad formed with the expanded sheet may advantageously have holes with diameters of 0.1-1.5 mm.
These holes may be formed, for example, by the perforation of the expanded sheet using a needle roller machine.
The surface of the pad which is to be in contact with the food may advantageously be covered with a film of opacified plastics material in order to mask from view the liquid held in the pad.
The use of a pad of this type in the production of a tray according to patent FR 2 688 474 is particularly advantageous.
The advantages of the tray according to the invention will become clear from the detailed description of some embodiments thereof, given below by way of non-limiting example with reference to the appended drawings, in which: